U.S. authorities are continuing to look for a motive behind Friday's deadly shooting at a Florida airport amid reports from relatives of suspected gunman Esteban Santiago that he was mentally traumatized by his 11 months of fighting in the U.S. war in Iraq.

Santiago was charged Saturday with an act of violence at an international airport resulting in death. If convicted of the charge, Santiago could face the death penalty or possibly life in prison for the attack that killed five people. He is scheduled to make his first court appearance in Florida on Monday.

"Not everyone has the same reaction when they return from war," his brother, Bryan Santiago, told CNN. "Some are better, and some, not so much."

According to their mother, Esteban Santiago, now 26, said that he was significantly affected when he saw a bomb explode next to two of his friends while he served in Iraq during most of 2010 and early 2011.

Bryan Santiago also said his brother requested psychological help but received little assistance late last year after walking into a local Federal Bureau of Investigation office in Anchorage, Alaska in an agitated state. He made incoherent statements and told agents he had been hearing voices directing him to join the terrorist group Islamic State.

Santiago was carrying a pistol magazine loaded with bullets at the time, but said he had left his gun in his vehicle.

As is their practice, the FBI agents took his weapon.

Santiago was taken for a mental health evaluation and investigated, but authorities found no wrongdoing. They returned his gun to him in December, but it is not clear if that pistol was used in the airport shootings, in which Santiago is accused of killing five people and wounding another six.

"How is it possible that the federal government knows, they hospitalize him for only four days, and then give him his weapon back?" Bryan Santiago said.

His uncle, Hernan Rivera, said, "Only thing I could tell you was when he came out of Iraq, he wasn't feeling too good."

U.S. Defense Secretary Ash Carter told NBC's "Meet the Press" show on Sunday that the country has not done enough to help veterans traumatized by war when they return home.

"We keep learning more about how to deal with this kind of illness," Carter said. "We're gonna learn more and we have to do more absolutely." He described post-traumatic stress disorder as "the so-called invisible wounds of war ... something we do take seriously and we have to take seriously."

But FBI officials have not ruled out a terrorism connection and say they do not know what motivated the shooter's barrage of gunfire in a baggage claim area at the Fort Lauderdale international airport. The gunman retrieved his weapon from checked luggage after a flight from Alaska.

Lead FBI investigator George Piro said Saturday, "We have not identified any triggers that would have caused this attack. We're pursuing all angles on what prompted him to carry out this horrific attack." Piro is the FBI agent who interrogated Saddam Hussein after the former Iraqi leader was captured by U.S. forces in 2003.

On Saturday, officials said at a news conference that Santiago traveled "specifically" to Fort Lauderdale to carry out the attack but do not know why he targeted the Florida city, a prime tourist destination. Authorities questioned him for several hours, until early Saturday, and said he was cooperating with investigators.